DOGUBAYAZIT, Turkey — Preliminary tests showed five more people in Turkey have been infected with the deadly strain of bird flu that already killed two teenage siblings, officials said Monday as Indonesia and China each reported a new case.

The new results raise the number of human cases in Turkey to 15, although most have not yet been confirmed by the World Health Organization.

However, a WHO official said Turkish patients appear to be catching the disease from infected domestic birds, the normal path of the disease, and not from each other.

But he warned that the chance that bird flu may mutate into a dangerous form transmitted from person to person increases with every new human infection.

Two teenagers from the same family died of bird flu last week. They were the first fatalities from the H5N1 strain of the virus outside East Asia, where 74 people have been killed by H5N1 since 2003.

The cases are turning up in Turkish towns and villages hundreds of miles apart, in every section of the country except the west. Turkish officials said they are near wetlands on the paths of migratory birds, which have been carrying the disease from country to country.

Indonesian authorities reported Monday that a 39-year-old man with a history of contact with poultry had died of bird flu, according to preliminary tests.

In China, authorities said local tests showed that a 6-year-old boy in stable condition at a central China hospital has tested positive for the H5N1 strain. Poultry at the boy’s home died before he fell ill, the official Xinhua News Agency reported.

Turkish labs have detected H5N1 in five new cases discovered in four provinces in eastern and central Turkey, as well as the Black Sea coast, Turkish officials said.

Ten people earlier tested positive for H5N1 in tests in Turkish labs, four of which have been confirmed by WHO.

In addition, more than 60 people with flu-like symptoms who came in close contact with fowl had been hospitalized around the country by Monday and were being tested, officials said.

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Scientists worry virus will mutateHealth officials are watching the disease’s spread and development for fear it could mutate into a form easily transmitted between people and spark a pandemic. WHO labs also are watching for genetic changes in the virus that could allow it to move from human to human.

“The more humans infected with the avian virus, the more chance it has to adapt,” Guenael R.M. Rodier, a senior WHO official for communicable diseases, said during a visit to Dogubayazit, a largely Kurdish town bordering Iran where three children have died.

Two siblings in the eastern city of Van died of the H5N1 strain last week. A third sibling also died in Van of bird flu, but a WHO lab has not yet confirmed H5N1 as the cause.

The doctor who treated the three children in Van said they probably contracted the illness by playing with dead chickens. The fourth case confirmed by the WHO also involved a person in close contact with fowl.

“It’s clear that the virus is well-established in the region,” Rodier said. “The front line between children and animals, particularly backyard poultry, is too large,” he said, adding that contact between poultry and people must be minimized.

Playing with dead birdsOn Monday, a fourth sibling was released from Van hospital after tests indicated he did not have the disease. Six-year-old Ali Hasan Kocyigit, the family’s only surviving child, left the hospital in his uncle’s arms, shyly gazing at cameras and journalists waiting outside.

Two preliminary H5N1 cases reported Sunday in Ankara, about 600 miles west of Van, involved boys aged 5 and 2 who apparently caught the virus while playing with gloves their father used to handle two dead wild ducks.

An 8-year-old girl hospitalized in Van with what Turkish labs showed was H5N1 apparently contracted the virus by hugging and kissing dead chickens.

On Monday, Health Minister Recep Akdag arrived with WHO officials in Dogubayazit, where many of the cases have originated.

“If as a community, we take the necessary measures and educate (people) we can in a short period of time combat this,” Akdag said. “We will manage to slow its progress.”

He said, however, that because Turkey was on the path of migratory birds, the country would continue to be at risk in years to come, and urged people to abandon raising poultry in backyards.

“The earlier we realize this, the earlier we will be rid” of bird flu, he said.

Akdag climbed up a snowy hill to visit Zeki Kocyigit, whose three children died of the disease. As he left, villagers shouted complaints about a lack of doctors.

Health officials believe the best way to fight the spread of bird flu is the wholesale destruction of poultry in the affected area. But they often run into problems in rural areas such as Dogubayazit, where villagers resist turning in their animals.